Interferometric Synthetic Aperture Radar (InSAR) is a technique for using radar to measure displacements in objects. For example, InSAR has been used for topographic mapping, measuring ground displacements, and for measuring the stability of coal mines. Synthetic Aperture Radar (SAR) typically uses a radar antenna mounted to a moving platform to capture readings of phase and amplitude at different positions of the antenna with respect to an object. SAR can capture 2-dimensional (2D) or 3-dimensional (3D) information based on how the antenna is mounted. For example, one technique samples radar returns in the horizontal dimension by moving the antenna along a horizontal rail and vertical returns by moving the antenna along a vertical rail. Depth information is derived by range synthesis of the radar signal and bandwidth. The radar antenna transmits continuous waves or pulsed waveforms at each rail position, recording the return of the reflected signal at each position to create an image of the object. This configuration is used to capture an initial reference image of the object, and later to capture a second image after the object has changed or is suspected of changing. A pixel-by-pixel complex cross product of the two images produces an interferogram that highlights differences in the two images as changes in the phase of the reflected radar signal that are converted to displacements.
Transportation corridors and residential areas bordered by rock faces and steep slopes face a constant threat from rock fall and landslides. For example, rocks may fall onto a highway interfering with the use of the highway and possibly even causing injury. Department of transportation authorities in mountainous states are frequently faced with the daunting challenge of mitigating rock fall from steep canyon walls adjacent to busy highways. The identification of rock fall threat and its mitigation (e.g., stabilization of rock or its removal) is time critical to minimize any threat to traffic safety and to ensure that any mitigation activity is scheduled during low traffic periods to minimize flow disruptions.
In an attempt to mitigate these threats, transportation and municipal authorities typically rely upon in-situ techniques to ascertain the location and extent of the threat. For example, personnel typically scale the rock face or slope and manually check the conditions. These techniques are rudimentary and involve visual inspection of rock faces to identify coarse movements of rock face segments. Transportation authorities have applied optical measurements using reflective targets that workers install in advance at various places on the surface of the rock wall surface. For example, laser-based Electronic Distance Measurement (EDM) can be used to monitor various points on the rock face. However, such techniques lack displacement resolution and fail to provide a complete picture of the rock face with the potential to miss problem areas that are not specifically measured.
In addition, all of these techniques involve personnel working on the rock face or slope, and therefore are labor-intensive, time-consuming, and dangerous. Once the transportation authority decides to stabilize or scale (i.e. remove) rock from an area, trained personnel are then utilized to either climb up or rappel from the top of the wall to reach the hazard area. This process is often inaccurate, hazardous to maintenance personnel, time consuming, and expensive.